The present invention relates generally to medical imaging. More specifically, the present invention relates to gating of an image scan to improve calcium scoring of a patient's heart and coronary arteries.
CT scanning of the heart is an increasingly common procedure to obtain information about the presence of calcification in the coronary arteries. Unfortunately, two body motions can interfere with the quality of the images obtained by the CT scanner: the heart motion and the patient's breathing motion. A normal heart scan takes about 20 seconds and to reduce the effect of the breathing motion, the patient is generally asked to hold their breath to eliminate the breath motion. The heart motion, on the other hand, cannot be readily eliminated and can lead to blurring, introduction of artifacts into the images, and misregistration.
A common procedure to reduce the heart motion is gating. As is described in U.S. Pat. Nos. 6,370,217 B1 and 6,243,437 to Hu et al., the motion of the heart is fastest during systole and relatively motionless during diastole. Prospective gating methodologies use an electrocardiograph signal (ECG) to predict the time of the diastole such that the CT scanner can be activated to obtain an image during the relatively motionless diastole period. A major issue with prospective gating in subjects with irregular heart beats is that the trigger can only be set to acquire data after the R-wave. If the following beat is short, the data acquisition may overlap the next systolic period. Retrospective gating, on the other hand, uses the electrocardiograph signal to retrospectively find motionless points in the heart cycle to select the image slice. In retrospective gating, the ECG signal information can be used, in retrospect, to select the slice images that were acquired during the diastole. The heart moves through a cycle in somewhat under a second, and a scanners generally take from a quarter second to a half second to acquire the information for each slice, thus it is possible to select from a number of slices for each cardiac cycle.
There are two major issues with retrospective gating. The first is that while reconstruction at finer intervals than the whole acquisition cycle does not increase the radiation dose to the subject to produce the extra images, the overlap of the scanned volume and the fact that the scanner's x-ray tube is continuously on (instead of being turned off during the parts of the cardiac cycle that are not of interest) increase the radiation dosage. The second problem is that gating from an ECG signal requires the placement of electrodes on the subject and testing to confirm that their placement is adequate. In a busy screening or diagnostic practice the added steps can decrease utilization and negatively affect the economics of the imaging operation.
There are various shortcomings in existing software for retrospective gating. When the operator is performing the selection of slices, there is no real time feedback as to the adequacy of the selection. Information as to the length of the cardiac cycle during the study, convenient ways to ascertain whether it changed during the study, and measurement of any one cycle are also not readily available. Except for manually adjusting each slice (there can be 350–500 slices in a study), there is no way to account for changes in the cardiac cycle. All of these contribute to decreasing the certainty with which a particular coronary calcium score is known, and to increasing the variability of the resulting calcium scores.
Consequently, what is needed are improved methods and software for generating a reconstructed projection image of the patient's heart which more fully utilizes the information content of the acquisition cycle, so that less of the increased dose is wasted or thrown out. Additionally, what is also needed are methods and software that can gate an image scan without the use of an ECG signal.